The present invention relates generally to implantable metallic medical devices. More specifically, the present invention relates to implantable medical devices, including, for example, surgical and endoluminal vascular grafts, stent grafts, skin grafts, shunts, bone grafts, surgical patches, non-vascular conduits, valvular leaflets, filters, occlusion membranes, sphincters, artificial tendons and ligaments. More specifically, the present invention relates to implantable medical grafts fabricated of metallic or pseudometallic films of biocompatible materials having a plurality of microperforations passing through the film. The plurality of microperforations may serve multiple purposes, including, for example, permitting geometric deformation of the film, imparting a fabric-like quality to the film, and imparting flexibility to the film. The term “fabric-like” is intended to mean a quality of being pliable and/or compliant in a manner similar to that found with natural or synthetic woven fabrics.
The inventive implantable grafts are fabricated entirely of self-supporting films made of biocompatible metals or biocompatible pseudometals. Heretofore in the field of implantable medical devices, it is unknown to fabricate an implantable medical device that comprises a graft at least as one of its elements, such as a stent graft, entirely of self-supporting metal or pseudometal materials. As used herein the term “graft” is intended to indicate any type of device or part of a device that comprises essentially a material delimited by two surfaces where the distance between said surfaces is the thickness of the graft and that exhibits integral dimensional strength and that has microperforations that pass through the thickness of the graft. The inventive grafts may be formed in planar sheets, toroids, and in other shapes as particular applications may warrant. However, for purposes of illustration only, the present application will refer to tubular grafts. For purposes of this application, the terms “pseudometal” and “pseudometallic” are intended to mean a biocompatible material which exhibits biological response and material characteristics substantially the same as biocompatible metals. Examples of pseudometallic materials include, for example, composite materials and ceramics. Composite materials are composed of a matrix material reinforced with any of a variety of fibers made from ceramics, metals, carbon, or polymers.
When implanted into the body, metals are generally considered to have superior biocompatibility than that exhibited by polymers used to fabricate commercially available polymeric grafts. It has been found that when prosthetic materials are implanted, integrin receptors on cell surfaces interact with the prosthetic surface. The integrin receptors are specific for certain ligands in vivo. If a specific protein is adsorbed on a prosthetic surface and the ligand exposed, cellular binding to the prosthetic surface may occur by integrin-ligand docking. It has also been observed that proteins bind to metals in a more permanent fashion than they do to polymers, thereby providing a more stable adhesive surface. The conformation of proteins coupled to surfaces of most medical metals and alloys appears to expose greater numbers of ligands and preferentially attract endothelial cells having surface integrin clusters to the metal or alloy surface relative to leukocytes. Finally, metals and metal alloys exhibit greater resistance to degradation of metals relative to polymers, thereby providing greater long-term structural integrity and stable interface conditions.
Because of their relatively greater adhesive surface profiles, metals are also susceptible to short-term platelet activity and/or thrombogenicity. These deleterious properties may be offset by administration of pharmacologically active antithrombogenic agents in routine use today. Surface thrombogenicity usually disappears 1-3 weeks after initial exposure. Antithrombotic coverage is routinely provided during this period of time for coronary stenting. In non-vascular applications such as musculoskeletal and dental, metals have also greater tissue compatibility than polymers because of similar molecular considerations. The best article to demonstrate the fact that all polymers are inferior to metals is van der Giessen, W J. et al. Marked inflammatory sequelae to implantation of biodegradable and non-biodegradable polymers in porcine coronary arteries, Circulation, 1996:94(7):1690-7.
Normally, endothelial cells (EC) migrate and proliferate to cover denuded areas until confluence is achieved. Migration, quantitatively more important than proliferation, proceeds under normal blood flow roughly at a rate of 25 μm/hr or 2.5 times the diameter of an EC, which is nominally 10 μm. EC migrate by a rolling motion of the cell membrane, coordinated by a complex system of intracellular filaments attached to clusters of cell membrane integrin receptors, specifically focal contact points. The integrins within the focal contact sites are expressed according to complex signaling mechanisms and eventually couple to specific amino acid sequences in substrate adhesion molecules. An EC has roughly 16-22% of its cell surface represented by integrin clusters. Davies, P. F., Robotewskyi A., Griem M. L. Endothelial cell adhesion in real time. J. Clin. Invest. 1993; 91:2640-2652, Davies, P. F., Robotewski, A., Griem, M. L., Qualitative studies of endothelial cell adhesion, J. Clin. Invest. 1994; 93:2031-2038. This is a dynamic process, which implies more than 50% remodeling in 30 minutes. The focal adhesion contacts vary in size and distribution, but 80% of them measure less than 6 μm2, with the majority of them being about 1 μm2, and tend to elongate in the direction of flow and concentrate at leading edges of the cell. Although the process of recognition and signaling to determine specific attachment receptor response to attachment sites is incompletely understood, availability of attachment sites will favorably influence attachment and migration. It is known that materials commonly used as medical grafts, such as polymers, do not become covered with EC and therefore do not heal after they are placed in the arteries. It is therefore an object of this invention to replace polymer grafts with metal grafts that can potentially become covered with EC and can heal completely. Furthermore, heterogeneities of materials in contact with blood flow are preferably controlled by using vacuum deposited materials.
There have been numerous attempts to increase endothelialization of implanted medical devices such as stents, including covering the stent with a polymeric material (U.S. Pat. No. 5,897,911), imparting a diamond-like carbon coating onto the stent (U.S. Pat. No. 5,725,573), covalently binding hydrophobic moieties to a heparin molecule (U.S. Pat. No. 5,955,588), coating a stent with a layer of blue to black zirconium oxide or zirconium nitride (U.S. Pat. No. 5,649,951), coating a stent with a layer of turbostratic carbon (U.S. Pat. No. 5,387,247), coating the tissue-contacting surface of a stent with a thin layer of a Group VB metal (U.S. Pat. No. 5,607,463), imparting a porous coating of titanium or of a titanium alloy, such as Ti—Nb—Zr alloy, onto the surface of a stent (U.S. Pat. No. 5,690,670), coating the stent, under ultrasonic conditions, with a synthetic or biological, active or inactive agent, such as heparin, endothelium derived growth factor, vascular growth factors, silicone, polyurethane, or polytetrafluoroethylene, U.S. Pat. No. 5,891,507), coating a stent with a silane compound with vinyl functionality, then forming a graft polymer by polymerization with the vinyl groups of the silane compound (U.S. Pat. No. 5,782,908), grafting monomers, oligomers or polymers onto the surface of a stent using infrared radiation, microwave radiation or high voltage polymerization to impart the property of the monomer, oligomer or polymer to the stent (U.S. Pat. No. 5,932,299). However, all these approaches do not address the lack of endothelialization of polymer grafts.
It is, therefore, desirable to fabricate the inventive graft of metallic and/or pseudometallic materials. The inventive metal devices may be fabricated of pre-existing conventional wrought metallic materials, such as stainless steel or nitinol hypotubes, or may be fabricated by thin film vacuum deposition techniques. In accordance with the present invention, it is preferable to fabricate the inventive implantable devices by vacuum deposition. Vacuum deposition permits greater control over many material characteristics and properties of the resulting formed device. For example, vacuum deposition permits control over grain size, grain phase, grain material composition, bulk material composition, surface topography, mechanical properties, such as transition temperatures in the case of a shape memory alloy. Moreover, vacuum deposition processes will permit creation of devices with greater material purity without the introduction of large quantities of contaminants that adversely affect the material, mechanical or biological properties of the implanted device. Vacuum deposition techniques also lend themselves to fabrication of more complex devices than those susceptible of manufacture by conventional cold-working techniques. For example, multi-layer structures, complex geometrical configurations, extremely fine control over material tolerances, such as thickness or surface uniformity, are all advantages of vacuum deposition processing.
In vacuum deposition technologies, materials are formed directly in the desired geometry, e.g., planar, tubular, etc. The common principle of vacuum deposition processes is to take a material in a minimally processed form, such as pellets or thick foils, known as the source material and atomize them. Atomization may be carried out using heat, as is the case in physical vapor deposition, or using the effect of collisional processes, as in the case of sputter deposition, for example. In some forms of deposition, a process, such as laser ablation, which creates microparticles that typically consist of one or more atoms, may replace atomization; the number of atoms per particle may be in the thousands or more. The atoms or particles of the source material are then deposited on a substrate or mandrel to directly form the desired object. In other deposition methodologies, chemical reactions between ambient gas introduced into the vacuum chamber, i.e., the gas source, and the deposited atoms and/or particles are part of the deposition process. The deposited material includes compound species that are formed due to the reaction of the solid source and the gas source, such as in the case of chemical vapor deposition. In most cases, the deposited material is then either partially or completely removed from the substrate, to form the desired product.
A first advantage of vacuum deposition processing is that vacuum deposition of the metallic and/or pseudometallic films permits tight process control and films may be deposited that have regular, homogeneous atomic and molecular pattern of distribution along their fluid-contacting surfaces. This avoids the marked variations in surface composition, creating predictable oxidation and organic adsorption patterns and has predictable interactions with water, electrolytes, proteins and cells. Particularly, EC migration is supported by a homogeneous distribution of binding domains that serve as natural or implanted cell attachment sites, in order to promote unimpeded migration and attachment.
Secondly, in addition to materials and devices that are made of a single metal or metal alloy, henceforth termed a layer, the inventive grafts may be comprised of a layer of biocompatible material or of a plurality of layers of biocompatible materials formed upon one another into a self-supporting multilayer structure because multilayer structures are generally known to increase the mechanical strength of sheet materials, or to provide special qualities by including layers that have special properties such as superelasticity, shape memory, radio-opacity, corrosion resistance etc. A special advantage of vacuum deposition technologies is that it is possible to deposit layered materials and thus films possessing exceptional qualities may be produced (cf., H. Holleck, V. Schier: Multilayer PVD coatings for wear protection, Surface and Coatings Technology, Vol. 76-77 (1995) pp. 328-336). Layered materials, such as superstructures or multilayers, are commonly deposited to take advantage of some chemical, electronic, or optical property of the material as a coating; a common example is an antireflective coating on an optical lens. Multilayers are also used in the field of thin film fabrication to increase the mechanical properties of the thin film, specifically hardness and toughness.
Thirdly, the design possibilities for possible configurations and applications of the inventive graft are greatly enhanced by employing vacuum deposition technologies. Specifically, vacuum deposition is an additive technique that lends itself toward fabrication of substantially uniformly thin materials with potentially complex three dimensional geometries and structures that cannot be cost-effectively achieved, or in some cases achieved at all, by employing conventional wrought fabrication techniques. Conventional wrought metal fabrication techniques may entail smelting, hot working, cold working, heat treatment, high temperature annealing, precipitation annealing, grinding, ablation, wet etching, dry etching, cutting and welding. All of these processing steps have disadvantages including contamination, material property degradation, ultimate achievable configurations, dimensions and tolerances, biocompatibility and cost. For example conventional wrought processes are not suitable for fabricating tubes having diameters greater than about 20 mm diameter, nor are such processes suitable for fabricating materials having wall thicknesses down to about 5 μm with sub-μm tolerances.
While the inventive self-supporting metal or pseudometal graft may be fabricated of conventionally fabricated wrought materials, in accordance with the best mode contemplated for the present invention, the inventive graft is preferably fabricated by vacuum deposition techniques. By vacuum depositing the metal and/or pseudometallic film as the precursor material for the inventive graft, it is possible to more stringently control the material, biocompatibility and mechanical properties of the resulting film material and graft than is possible with conventionally fabricated graft-forming materials. The inventive self-supporting graft may be used alone, i.e., the whole implantable device may be made of a single graft, or it may be a part of a structure where the graft is used in conjunction either with other grafts, or in conjunction with other structural elements, such as scaffolds, stents, and other devices. The term “in conjunction” may mean actual connection, such as that made by welding, fusing, or other joining methods, as well as being made from the same piece of material by forming some area of the piece into a graft and some other area of the piece into another member or part of the device.